fea analysis of the dynamic response of orthogonal testing

1. Cross-Axis motion
during Vibration testing
Is it significant ?
4/18/2018 rev A
Don Blanchet
3B Associates
dwb3298@verizon.net
Prepared for: 3BA Burlington , MA
Image courtesy of
Vibration data.com
Large spacecraft on
An electrodynamic
Shaker.

2. Vibration testing
 Almost always specified to be
performed in 3 mutually perpendicular
primary directions. X, Y, & Z axis tests.
 The intensity and frequencies are
controlled by accelerometers located
on the mounting fixture close to the
shaker table mounting surface.

3. Cross Axis response ?
 Overturning moments are present in
most all tested equipment.
 This results in responses in the axes
perpendicular to the primary axis
being excited.
 The nature of these responses must
be understood for they may represent
a worst case response for all test
directions of the UUT.

4. Laser electronics assembly
8 inch diameter
Aluminum housing
sensor
Mounting surface

5. FEA mesh

6. Modal analysis f1 = 588hz

7. Modal analysis f5 = 1235 hz
Sensor resonance

8. Z-axis vibration test
in-line Z response
Q=58
Base input
ASD/PSD

9. Z-axis vibration test
cross axis responses
Base input
ASD/PSD
Q=4
Q=10

10. Summary – Q Factors
Z-INPUT
X-cross
axis
RESPONSE
Z-INPUT
Y-cross
axis
RESPONSE
Z-INPUT
Z-in line
test
RESPONSE
X-INPUT
X-in line
test
RESPONSE
F= 1235 HZ 4 10 58
F= 588HZ 32

11. Conclusions
 X,Y,Z orthogonal vibration testing does
not necessarily result in orthogonal
peak responses in the unit under test.
 “cross axis” responses can be a
significant fraction of the in-line
direction responses.
 The resultant cumulative fatigue life
calculations must include all
responses.